The invention relates to circuitry for precisely detecting the end of a tape in an audio or video tape transport apparatus, and for initiating the stopping of tape movement by conventional circuits upon the detection of an approaching end.
In the field of audio and video tape recording, it is common practice to utilize various servo circuits to control the movement of tape between supply and takeup reels of the corresponding tape transport. Typical of such servo circuits are reel servo circuits and capstan servo circuits which are utilized in various fashions to control tension in the tape, as well as to control the movement and speed of the tape while performing the various operations of recording, reproducing and shuttling of the tape from reel to reel. Included in such movement controlling servo circuits is a circuit for determining the approaching end of the tape and for then decelerating the tape in response to such determination, thereby bringing the tape to a stop as rapidly as possible. In addition to stopping quickly, it also is desirable to start the deceleration process as close to the end of the tape as possible so as to reach the end as rapidly as possible. In sophisticated broadcast tape recorders, it is imperative that the movement of the tape, and particularly the shuttling process, be performed at very high speeds which, in turn, requires that the precise end of the tape is known, whereby the maximum shuttle speed can be maintained as long as possible before reaching the end.
However, notwithstanding the desire for speed, it is equally important that the end of the tape which is wrapped about, or secured to, the hub of the reel is not pulled from the hub. That is, either end of the tape must be reached as rapidly as possible when at shuttle speed, but without detaching the tape end from the reel. This is particularly important in tape transports which utilize enclosed cassettes as the supply of tape, wherein the ends of the tape are permanently attached to respective supply and takeup reels within the cassette. Pulling the tape end from the reel would make the cassette inoperative and useless.
Typical of "end-of-tape" detectors presently available, are those systems which measure the amount of tape on a reel utilizing the principle that the rate at which the reel rotates as the tape is wound or unwound therefrom varies in proportion to the amount of tape on the reel. In such systems, a first series of pulses are provided which indicate the rotational velocity of the reel, and a second series of pulses are provided for indicating the linear velocity of the tape.
In one embodiment of this type of end-of-tape detection technique, the number of pulses generated by the linear tape movement are counted during each interval of time between the pulses generated by the rotating reel. The accumulated counts vary in proportion to the amount of tape remaining on the respective reel being unwound, and thus can be used to indicate the approaching end-of-tape.
In a second embodiment of such a technique, the respective successions of pulses generated by the reel rotation and by the linear movement of the tape, are utilized in a different fashion. More particularly, a counter, responsive to the tape movement pulses and the reel rotation pulses, develops a signal representing the ratio of longitudinal tape speed to reel rotational speed. In addition, a tape pack diameter selector produces a preselected ratio of longitudinal tape speed to reel rotational speed corresponding to a desired terminal tape pack diameter. A comparator compares the signal with the preselected ratio, and includes a storage register which produces a control signal which may, for example, decelerate the tape driving means when the signal and the preselected ratio achieve at least one and generally a plurality of value matches in succession.
The end-of-tape detector systems of previous discussion provide generally sophisticated means for determining the end of the tape and for decelerating the tape to a stop upon sensing the approaching end. However, systems such as those above which calculate the distance to the approaching end of the tape, or otherwise use pulses derived from a capstan or idler guide which are indicative of tape movement, require the use of a large safety cushion in terms of time or revolutions of the reel, in order to insure that the end of the tape is not ripped from the reel hub, as will happen if the tape end is actually closer than the calculated distance. Such a condition readily is possible in tape transports where capstans, which impart movement to the tape, can slip when pulling the tape, resulting in an erroneous number of linear pulses being generated by the capstan. This in turn erroneously defines the distance to the end of the tape, and inherently indicates there is more tape on the reel than is actually true. Thus, in such situations, the tape is pulled off the hub, which prohibits further use of the damaged cassette.
As mentioned above, speed in moving tape is very important and, accordingly, it is highly desirable to be able to drive a tape at maximum shuttle speeds to the very end of the tape before coming to a stop. However, it also is imperative that the tape not be detached from the supplying reel, particularly in a tape transport employing a cassette configuration.
The present invention provides a simple yet very sophisticated system for determining the precise end of a tape, while overcoming the disadvantages of previous mention. That is, the end-of-tape determined by the system is not affected by capstan slippage or other tape movement parameters which can negatively affect typical end-of-tape detectors presently being used. To this end, once the end of the tape or nearly the end of the tape is located, the corresponding position is remembered and stored as a preset end-of-tape value. When tape is wound back onto the reel, the number of revolutions accumulated also are stored. Thereafter, if the direction of tape changes again, the revolutions of the reel which now is supplying the tape are counted down until the previously stored end-of-tape value corresponding to the end, or nearly the end of tape, is reached. The value is used to determine a point where a conventional servo circuit is enabled to first rapidly decelerate the tape to a safe speed just before the end-of-tape, whereupon the servo circuit then precisely stops the tape movement in response to detecting the true end-of-tape.
More particularly, reel tachometers are mechanically coupled in conventional fashion to respective supply and takeup reels. A "counter means" include supply and takeup up/down counters coupled to respective reel tachometers, wherein the counter means in essence count each revolution of the respective reels to provide continuous data as to the distance from the "end-of-tape" (on the supply reel), or the "beginning-of-tape" (on the pickup reel). Although the distinction between the terms end-of-tape and beginning-of-tape is made briefly above, the term "end-of-tape" is commonly used in the art and thus is used in the description hereinafter, to indicate either end of the tape. The reel tachometers provide the one or more pulses indicative of each of the revolutions of the respective reels. The system includes a microprocessor/memory means wherein, in the preferred embodiment, the memory defines in part the "counter means" of previous mention in which the preset value indicative of the true end-of-tape is stored. Under control of selected software, the collective "counter means" accumulate and also store the tach pulses corresponding to the revolutions of each reel as the tape is being moved between the supply and takeup reels.
Thus, by way of example, if a full supply reel is to be threaded on a tape transport, once the takeup reel is threaded, or the cassette is loaded, the junction between tape and clear leader is detected by an associated sensor means to define the true end-of-tape. The associated takeup counter means, i.e., memory, is set to a zero value, or to some other selected reference value, at the exact point where the clear leader is spliced to the tape. Thereafter, every revolution of the takeup reel increments the takeup counter means by one or a selected number of counts as the tape is wound onto the takeup reel, and decrements the takeup counter by one or the selected number of counts, when tape subsequently is pulled off the takeup reel and returned to the supply reel. Accordingly, the true end-of-tape on the takeup reel is precisely known and is continuously updated and stored in the associated microprocessor memory via the takeup counter while winding or unwinding tape. The distance to the end-of-tape is stored in terms of the number of reel revolutions; that is, in terms of the number of reel tach pulses indicative of the exact number of revolutions.
Likewise, when the supply reel is threaded and the clear leader-to-tape junction is detected by an associated sensor means, the corresponding end-of-tape is precisely known, and the memory associated with the supply counter means is preset. Thus, the distance to the true end-of-tape also continuously is known while winding tape back onto, or from, the supply reel.
When pulling tape from a reel the microprocessor means continuously monitors the memory of the counter means, and thus is aware of an approaching end-of-tape from either reel. Upon detecting an approaching end-of-tape, the microprocessor means initiates the deceleration of the tape, via a conventional transport tape speed control circuit, to a selected safe speed a preselected short distance prior to the true end-of-tape. The transport then pulls the tape the short distance at the safe speed until the associated sensor means detects the clear leader-to-tape junction, that is, the true end-of-tape. A control signal is supplied to the conventional control circuit to immediately stop the tape. The memory of he counter means is reset to the reference value corresponding to the true end-of-tape, as previously discussed.
In the situation where a system is turned on with tape distributed on both reels, the counter means generally do not contain the preset reference values corresponding to the respective true ends-of-tape. Upon pulling tape in either direction, the transport control system relies upon the conventional prior art end-of-tape detection technique using, for example, the tape pack diameter and ratio of tape speed to reel rotational speed information of previous mention. At such time as tape is pulled to a true end-of-tape, or to nearly the end-of-tape, the end-of-tape detection technique reverts to the invention combination, as further described below .